1. Field of the Invention
The present invention relates to coating processes and apparatus and, more specifically, processes and apparatus for uniformly coating discrete substrates by application of a controlled volume of fluid per unit surface area of a substrate. The invention is useful for a wide variety of applications including the manufacture of electronic printed circuit boards, liquid crystal displays and electronic integrated circuits.
2. Background Art
Application of a fluid coating onto a substrate is a necessary process in the manufacture of a wide variety of items. For example, a photoresist composition is applied to a substrate to fabricate printed circuit boards, liquid crystal displays, integrated circuits and other products. A typical liquid-type photoresist composition comprises a photosensitive compound or a photoinitiator dissolved or suspended in an organic solvent composition. After application to a substrate and evaporation of any solvent carrier, the photoresist is selectively exposed through use of a photomask and actinic radiation. The photomask provides areas that are selectively opaque and transparent to the radiation, and thereby defines and transfers a pattern to the photoresist coating layer. The patterned resist is then developed, for example by application of a developer solution. After development of the resist coating, the patterned substrate surface may be selectively processed, such as chemically milled, plated or coated. See, generally, Coombs, Printed Circuits Handbook, ch. 11 (McGraw Hill, 3d ed. 1988), incorporated herein by reference. Chemical milling is performed by application of a suitable etchant solution. The etchant solution degrades only those portions of the substrate surface bared of photoresist. Similarly, the substrate can be treated with a suitable plating solution to deposit metal only on those surface areas bared of photoresist.
Other processes require application of a solder resist (soldermask) composition. In the manufacture of a printed circuit board, a solder resist functions to restrict solder deposit or flow to only those areas of a printed circuit board that are not covered by the resist. Providing a uniform coating layer can be challenging as the resist is typically applied after the board surface has been built up in a non-uniform fashion, for example, after fabrication of board circuitry. Applying a uniform coating layer across such an irregular surface is generally a difficult process.
Many coating processes, for instance spray coating and curtain coating, apply an excess of fluid to a substrate surface. See, for example, U.S. Pat. Nos. 4,544,623 and 4,143,190, incorporated herein by reference. Application of excess fluid results in waste as well as requiring clean-up or recovery steps. Additionally, if coating is to be restricted to only a portion of the substrate surface, some type of masking procedure is necessary.
Moreover, many coating methods expose fluid to an open atmosphere for an extended period of time as well as recirculate and/or reuse fluid that has been already exposed to other substrates. For instance, curtain, dip and roller coating processes all typically expose fluid to an open atmosphere for extended periods; dip coating immerses multiple substrates in the same coating vessel; and a curtain coating process typically recirculates and reuses excess applied fluid.
Such open exposure of fluid and fluid recirculation and reuse gives rise to notable problems. Volatile solvents evaporate from fluid exposed to an open atmosphere. Consequently, either the viscosity of the fluid will vary over time, thus compromising uniformity of the applied coatings, or regular and precise solvent additions must be made throughout the coating process. This latter approach is generally too burdensome for commercial applications, particularly in larger scale operations. Extensive solvent evaporation also poses safety and environmental concerns and requires a suitable venting system. Further, both exposure of fluid to an open atmosphere and fluid reuse results in rapid accumulation of contaminants in the fluid supply. This compromises uniformity of the applied coating and can be entirely unacceptable in many processes. While use of a clean room and scrupulous substrate pre-cleaning can alleviate fluid supply contamination to some degree, such measures are both expensive and burdensome.
Additionally, some coating processes are limited in the range of fluid viscosities that can be satisfactorily applied. For example, to maintain fluid sheet integrity in a curtain coating process, higher viscosity fluids are generally employed. Dip coating provides best results for lower viscosity fluid applications.
These shortcomings of prior coating systems pose significant limitations in the manufacture of many substrates and, more specifically, in the manufacture of printed circuit boards, liquid crystal displays and integrated circuits. Fabrication of these substrates generally requires the application of a highly uniform coating. This demand for coating layer uniformity increases directly with substrate performance requirements. Contamination of fluid supply, viscosity variations arising from solvent evaporation, and other problems of prior coating systems that compromise the integrity and uniformity of the coating layer impose real limits on substrate quality and performance. For example, increased circuit densification is continuously sought; this in turn requires image transfer with maximum resolution. Any irregularities in a photosensitive coating layer generally only serve to reduce resolution. Similarly, higher performance applications require plating processes where plating is strictly limited to specific substrate areas. Decreased resolution of an image patterned on a photosensitive coating can result in migration of a plating solution to substrate areas not intended for plating. Further, photoresists, soldermasks, etchant solutions and other fluids used in the manufacture of printed circuit boards, liquid crystal displays and integrated circuits can be quite expensive. Hence, application of excess fluid or other fluid waste is undesirable.
Another coating method is generally known as "slot coating" in reference to the horizontal, elongate fluid applicator orifice that is typically employed. See, for example, U.S. Pat. Nos. 4,696,885 and 4,938,994, both incorporated herein by reference. It is possible by a slot coating process to apply a generally fixed volume of fluid onto a substrate surface, thus providing advantages of fluid conservation as well as avoidance of excessive fluid dispersion throughout the application area. Further, it is possible by a slot coating process to maintain the fluid in a closed system until deposit of the fluid on a substrate. This overcomes problems of fluid contamination and viscosity variations inherent in "open" type coating systems.
For slot coating on discrete substrates, it may be preferred to apply a separate fluid volume to each substrate; that is, a separate start and stop of fluid flow for each substrate. This is referred to as an intermittent fluid application. While slot coating in general enables control of the width of a coating layer by variation of the length of the slot orifice, intermittent slot coating can further enable control of coating layer length by selectively stopping and/or starting fluid flow within the substrate perimeter. Thus, a perimeter area of the substrate can be left uncoated, enabling ease in handling during subsequent manufacturing steps. Terminating fluid flow just up to or within a substrate perimeter can also avoid fluid accumulation along substrate edges. Such accumulation results in an uneven coating layer. Further, there can be multiple starts and stops of fluid flow within the substrate perimeter, providing selectively coated substrate areas without use of a masking type procedure. Still further, intermittent coating can enable control of coating throughput so to match drying or other processing capacity.
To apply a uniform fluid layer by intermittent coating, a precise start and stop of flow fluid is required for each treated substrate. Dripping of excess fluid after flow termination, however, is a persistent problem. By depositing an excess of fluid on one portion of the coating layer, dripping compromises the layer's uniformity. Dripping of fluid after flow termination can result in several ways. Surface tension adheres fluid to the outer face of the fluid applicator; after flow termination, this fluid will drip onto the substrate surface. Fluid which has traveled past the flow control apparatus into the fluid applicator will also drip onto the substrate after flow has terminated. Further, precise flow cutoffs are particularly difficult with low viscosity fluids, i.e., fluids having a viscosity of less than about 300 centipoise (cps). Specifically, trailing and dripping of fluid after flow termination is common for low viscosity fluids.
For coating multiple substrate sides, prior slot coating processes generally provide positioning a substrate along a horizontal plane and downwardly applying fluid from a fluid applicator directly above the substrate surface; at a later time flipping the substrate to expose an uncoated surface; and then carrying out another fluid application step for the uncoated side. In addition to the time and expense necessitated by such a multiple step procedure, uniformity between coating layers on different substrates sides is compromised. If drying or other fluid layer processing is performed between fluid applications, the first applied coating layer will be exposed to more processing steps than a subsequently coated substrate side. Even if there are no intervening processing steps, the first coated side will be exposed to an open atmosphere longer than subsequently coated sides. Further, conventional double-sided slot coating processes effectively limit the number and type of suitable coating compositions. Compositions that are altered significantly by longer processing or exposure periods are generally unsuitable due to the consequential discontinuities of the coating layers on each substrate side.